DET([c, 3, 1; 2, -1, 2·c; 1, 4, c]) = 9·(c + 1)·(1 - c) ≠ 0 --> c ≠ ±1
[c, 3, 1; 2, -1, 2·c; 1, 4, c]^-1 = [c/(c^2 - 1), (3·c - 4)/(9·(c^2 - 1)), (6·c + 1)/(9·(1 - c^2)); 0, - 1/9, 2/9; 1/(1 - c^2), (4·c - 3)/(9·(c^2 - 1)), (c + 6)/(9·(c^2 - 1))]
[c/(c^2 - 1), (3·c - 4)/(9·(c^2 - 1)), (6·c + 1)/(9·(1 - c^2)); 0, - 1/9, 2/9; 1/(1 - c^2), (4·c - 3)/(9·(c^2 - 1)), (c + 6)/(9·(c^2 - 1))] * [5; 3; 6] = [2/(c + 1); 1; 2/(c + 1)]